Scientists are closer to understanding the mysterious "circadian" rhythm that governs sleep and wake after an experiment using fruit flies.

They bred flies missing a gene vital to the daily cycle - which made sleep deprivation have fatal consequences within hours.

But they managed to help the flies survive longer by kick-starting a process which protects cells from sleep deprivation.

Despite years of study, relatively little is known about the mechanism by which animals shut down for the night - or even precisely what biochemical processes are taking place while the body is asleep.

The effects of sleep deprivation - in both humans and flies - can be damaging.

A lot in common

Fruit flies are a handy substitute for humans because the two species share many of the same genes.

In the case of the circadian clock, flies have a gene called "cycle" which plays a vital role in maintaining a sleep/wake rhythm.

In humans the equivalent gene is called bmal1.

Flies bred lacking their cycle gene tended to die after missing only 10 hours of sleep, found scientists from the Neurosciences Institute in San Diego, California.

Scientists noticed that other genes in these mutated flies were less active after sleep deprivation.

These genes, which produce "heat shock" proteins, were artificially stimulated prior to sleep deprivation by raising the fly's environment to human body temperature.

Chaperones

As the processes involved in human sleep cycles are thought to operate on broadly the same terms, it is possible these "heat shock" proteins in humans may also influence recovery from the effects of sleep deprivation.

The proteins are cellular "chaperones" - preventing cells from suffering damage when they exposed to stresses such as excess heat, lack of nutrition - or sleep deprivation.

It may be that increased understanding of this process leads to treatments for patients suffering from this problem.

Certain death

Dr Paul Shaw, who led the research, published in the journal Nature, said: "It's been known for many years that the circadian clock can control sleep timing, but what these data show is that one of its key components plays another role in sleep.

"The role is so important that when that gene's gone, these animals will die from sleep loss."

Professor Bambos Kyriacou, from the University of Leicester, told BBC News Online: "What's important is that these are the first people to look at the molecular biology of sleep and sleep deprivation.

"These 'heat shock' proteins protect cells when they are under stress.

"Perhaps, if you can turn on these proteins in humans, it could be beneficial."